Extrusion is the process of forcing material through a die having an extrusion profile to form a product having a cross section that matches the extrusion profile. The length of the extruded product is determined by the amount of material forced through the die. A typical aluminum window frame may be fabricated from extruded rails and stiles. A typical rail or stile has a relatively complicated cross section including a plurality of arms extending from a common spine. Additionally, each of the arms may have a plurality of members extending therefrom. In the past as the extrusion profile became more complex, the speed of the extrusion process had to be reduced to maintain a high quality product.
A depiction of a typical extrusion die known in the art may be seen in FIG. 1. The prior art extrusion die, indicated generally by the numeral 210, generally includes a die body 212 having an upstream face 214 and a downstream face 216 with a cavity 218 extending toward the upstream face 214 from the downstream face 216. An extrusion profile 220 is cut from the upstream face 214 through the die body 212 to the cavity 218. A wall 222 parallel to the upstream 214 and downstream 216 faces extends between the extrusion profile 220 and the cavity 218. This wall 222 can also be referred to as the undercut 222 of the die 210. The depth of the extrusion profile 220 is referred to in the art as the die land or the die bearing 224. The die land or bearing 224 is the portion of the die 210 that the material contacts as it is forced through the die 210. Such contact causes friction that creates heat and negatively affects material flow.
The length of the bearing 224 and the length of the undercut 222 affect the strength of the die 210. The strength of the die 210 is important because the die 210 is subjected to high pressures and high temperatures during the extrusion process. If the material surrounding the extrusion profile 220 is weak, the quality of the product is negatively affected. To increase the strength of the die 210, a longer bearing 224 and a small undercut 222 may be used. A long bearing 224, however, decreases the speed of the die 210 because of the friction created by the long bearing 224.
Thus, it is desirable to minimize the length of the bearing so that the maximum extrusion speed may be achieved while maintaining adequate strength for the die. Maximizing extrusion speed is extremely important to the extrusion industry because a die may be used to create miles of product over its lifetime. Thus, even a small increase in extrusion speed yields large benefits to the manufacturer.
Another feature of known dies 210 is a cavity 230 formed in the upstream face 214 of the die 210 to facilitate consecutive billets. Consecutive billets are required when the desired length of the product is longer than the capacity of the extrusion processor. To allow consecutive billets, a cavity 230 is carved out of the upstream face 214 of the die 210 around the extrusion profile 220. When the ram of the extrusion processor approaches the upstream face 214 of the die 210, the billet is cut and a portion of the extrusion material remains in the cavity 230. When the billet is cut, the act of cutting creates a force that tends to pull the material remaining in the cavity 230 back out of the die 210. To prevent the material from being pulled entirely out of the cavity 230, the cavity 230 is relatively deep. The depth is such that the angle indicated by the numeral 232 is typically less than 45 degrees. The depth of the cavity 230 prevents the cutting force from pulling the material all the way out of the die 210. Once the material is cut, the ram is then pulled back and another billet is inserted. The new billet welds itself to the material left over in the cavity and the extrusion process is continued.
The depth of the cavity 230 negatively effects the performance of the extrusion die 210. When the angle 232 formed by a line normal to the upstream face 214 at the corner of the cavity 230 and a line taken through that corner and the corner of the extrusion profile 220 and the bottom 234 of the cavity 230 is less than 45 degrees, the flow through the die 210 is restricted. As the material is forced against the die 210 in the extrusion processor, areas of material are forced into the corners and essentially stay in the corners during the extrusion process. This area is known as a dead area of flow and is indicated generally by the numeral 236 in FIG. 1. The dead area 236 creates friction between the rest of the flow and itself. A deep cavity 230 causes an additional dead area to form, as indicated by the numeral 238. The deep cavity 230 also acts as an additional length of bearing where the flow may flow against the cavity walls, as indicated by the numeral 240. The additional friction created by the dead area 238 and the extra bearing 240 is undesirable because it creates heat which degrades the surface finish of the final product. To reduce the affects of friction, the extrusion processor is run at slower speeds.
To design such a conventional die, a die designer typically relies on a trial and error method. The success of the die design often depends on the knowledge and experience of the die maker. A die is currently manufactured by first determining the desired profile of the final extruded product. The profile is then cut out of the die body. When the die designer first cuts the profile, the designer intentionally leaves the bearing longer than desired so that bearing length may be removed, if needed, after a test run. The die is then placed in an extrusion processor and run through a series of tests. If the die functions properly, the die is then used to create final products. A problem with this method is that the bearing of the die has been left intentionally long and the die must be run at slow speeds.
If the designer discovers problems with the die during the test runs or desires a faster die bearing, the designer takes the die out of the processor and makes adjustments. The magnitude of these adjustments often depends on the knowledge and experience of the designer. One typical adjustment that may be made is the removal, or shortening of the bearing. The known methods for removing bearing are to shorten the entire bearing or to shorten a portion of the bearing to create a stepped bearing. Once this has been done, the die is repositioned and additional tests are performed. One problem with creating a stepped bearing is that a die having a stepped bearing forms a product with surface lines at the location of the bearing step. Such lines are undesirable and must be removed by a further process.
The re-configurations and tests are repeated until a satisfactory product and extrusion speed are attained. It should be noted that bearing length cannot be added back to the die after it has been removed. Thus, if too much bearing is removed, the die must be scrapped and the process repeated. For this reason, the die bearing is always left longer than necessary. The added length causes the extrusion processes to be run slower than possible. Even a knowledgeable die designer with significant experience typically requires approximately three tests to create a satisfactory die. The number of runs and the labor required to perfect the die undesirably increases the costs of forming the die.
The speed and temperature at which the extrusion press is operated are also determined by a similar trial and error method. When a new die is placed in the press, the apparatus is run at a initial test speed and temperature and the quality of the extruded product is examined. If the surface quality is acceptable, the speed of the machine is increased and the product checked again. When the product quality becomes unacceptable, the speed is backed down and the temperature is adjusted until a good quality product is produced. The knowledge and experience of the person operating the apparatus effects the length of this process. In addition to the amount of downtime experienced by the set up of the process, a significant amount of material is wasted performing the test runs. It is thus desirable to provide a method for designing an extrusion process that substantially reduces the amount of set up time and reduces test runs.
This method for setting up the extrusion press bases the press set up around the die characteristics. In other words, the die is designed first and then the extrusion press parameters are set based on the performance of the extrusion die. The result of this method is that the extrusion press is often not run at optimal extrusion conditions because the extrusion die has not been designed for optimal conditions. As such, it is desired in the art to provide a method for designing an extrusion process that first calculates the optimal press conditions and then designs the extrusion die based on these optimal conditions.